The invention relates generally to the sampling of moving fluids such as moving airflows, and more particularly to a sampling system that extracts samples of a moving fluid passing thereover.
Moving fluids such as airflows frequently must be sampled for a variety of flow monitoring applications. Such sampling may be performed to examine the ambient air for chemical, biological and/or radiological particulates. Other purposes may include inertial characteristics of the airflows, such as provided by pressure measurements.
A typical sampling system incorporates a housing having an inlet formed therein and a pump or fan. The inlet faces directly into the flowstream, and the fluid expands into a diffuser before being diverted to a collector. As fluid (e.g., air) moves over the housing, the pump draws the fluid into the housing through the inlet and toward the collector. The inlet and pump may be optimized for an expected set of external flow conditions. In particular, the system can be designed for appropriate pump power consumption and pump speeds under the expected fluid flow conditions.
However, if the fluid flow speed significantly exceeds the design parameters, the Bernoulli effect at the housing's inlet causes backpressure to develop in the housing. Bernoulli's principle concerns the relationship between static and dynamic pressures, such that P0=P+½?u2, where P0 represents stagnation or total pressure (of fluid being at rest), P is static pressure (parallel to fluid flow), ? is fluid density and u is fluid velocity.
As the fluid enters the housing, the velocity decreases, thereby increasing static pressure inside the housing. The difference between the internal housing static pressure and the external static pressure in the ambient stream represents the backpressure. As the backpressure increases within the housing, the pump must rotate faster than its design operational levels to draw the moving fluid into the collector. Such continued beyond-design operation may yield decreased pump efficiency and increased risk of motor overheating.